1. Field of the Invention
This invention relates to A.C. air ionizers, and more particularly relates to electrical apparatus and circuitry for measuring corona current in the ion stream being emitted from the discharging electrodes of such air ionizers in order to evaluate the efficiency thereof at any time.
Corona discharge electrodes operating on alternating current are utilized in a variety of air ionizing devices for generating both positive and negative ions, such as static eliminators or neutralizers, corotrons of electrostatic copiers, electrostatic precipitators, and the like. Typical electrical A.C. air ionizing devices are driven by high voltages ranging from about 3,000 volts to 10,000 volts at frequencies from about 50 Hz to 20 KHz by means of a high voltage transformer whose output is coupled to one or more sharp electrodes in the form of points or small diameter wire positioned in the proximity of ground. Corona current is only generated when the voltage on the discharge electrodes exceeds the corona onset level, and there is no corona current when the point voltage is below that threshold. The average corona current for a given corona electrode is a function of the voltage applied to the corona electrode.
A typical wave form for total current (capacitive current plus corona current) with respect to time is shown in FIG. 1, the capacitive (or displacement) current which is substantially sinusoidal in configuration being always present during operation and 90.degree. out of phase with the A.C. voltage applied to the discharge electrodes.
When the voltage applied to the discharge electrode exceeds the corona threshold level, the corona current will start flowing and continue to flow until the voltage on the electrode decreases below the threshold level. The pulse of corona current normally begins about the time that the sinusoidal waveform of the displacement or capacitive current reaches its peak value. The corona threshold or onset level is a function of the sharpness of the discharge electrode points, their distance from a proximity ground, the atmosphere of operation, and the degree of contamination of such discharge electrodes.
As is well known, corona electrodes tend to attract contamination from the environment, especially dust. As the electrodes become covered with contamination, their corona characteristics change--usually, the corona current diminishes, and the air ionizers become less efficient so as to become ultimately ineffective. This condition is quite difficult to detect, particularly in the case of air ionizers where a strong signal generated by the field from the corona electrodes via the capacitance of the air masks the signal generated by the ion flow itself.
2. Prior Art
In U.S. Pat. No. 4,757,422 to Bossard, a detector screen is interposed in the ion path within the housing at the ion exit port for the purpose of capturing some of the ions produced by the electrodes. The detection screen is of conductive material and held at virtually ground level by means of a low impedance operational amplifier circuit. In addition, a similar screen coupled directly to ground is positioned between the ion generating electrodes and the detection screen in order to terminate the electric field lines emanating from the electrodes.
However, this arrangement has the disadvantage that only air ionizers using fans or blowers to propel ions past the grounded screen into the environment can be employed. That is ionizers which do not use blowers or fans will have all generated ions captured by the shielding screens, hence being rendered ineffective. Secondly, for a shielding screen to be effective in eliminating the field influence, such a screen must be quite dense which will result in a significant portion of the ions generated being captured, even should a blower be utilized to extend the range by propelling ions. Thirdly, in the instance of static eliminators which use the field of the charged object for neutralization, the shielding screen again will mask the object field and inhibit ions from reaching the charged surface.